The present invention relates to a method for optimizing the combustion of fuel in a furnace used to heat a fluid. The furnace may be a conventional furnace used for generating steam in a power plant or a heater used to heat process fluids in a refinery or chemical plant. An optimizer controller of this type is illustrated in U.S. Pat. No. 3,184,686. The controller illustrated and described in this patent does not require any stack analysis equipment to derive data for optimizing the combustion process. Instead the controller reduces the excess air to an absolute minimum to achieve optimization of the combustion process. In this respect the controller seeks to find the point on a curve of fuel versus excess air where the fuel flow is minimum. This point is usually not the point of minimum excess air since if too little excess air is used some fuel remains unburned and the combustion process is less efficient. The optimizer allowed the outlet temperature of the fluid being heated in the furnace to control the fuel flow to the furnace and reduced the excess air until the fuel flow increased. When the fuel flow increased, the air flow was increased and then reduced until the fuel flow again increased. Thus, there was a possibility of the furnace smothering when the temperature of the fluid signaled for more fuel flow while the controller was attempting to reduce the excess air below the minimum required for complete combustion. In order to prevent this, the controller incorporated means for rapidly increasing the air flow whenever the excess air was being reduced and the fuel flow was suddenly increased.
In addition, the above controller comprised a separate analog control unit while today numerous refineries and chemical plants have large centralized computer installations. Thus, many of the functions performed by the controller can be provided by the centralized computer. Also, the means by which the controller sought the optimum excess air flow provided a rather erratic control with a possibility of smothering the furnace with an excess fuel flow.
Further, the system disclosed in the patent made no provision for controlling the air flow in relation to the fuel flow. Instead the patent attempted to reduce the air flow at a relatively slow rate while permitting the fuel flow to vary in response to the load on the furnace. If a point was reached where the fuel flow increased the system assumed this was due to incomplete combustion and increased the air flow at a rapid rate. While this type of optimization was successful under substantially steady state conditions, it will not respond to varying fuel flows caused by varying loads on the furnace. Likewise, the system also will not respond when a furnace is burning various amounts of different types of fuels. For example, in refineries, as natural gas becomes more scarce, more fuel oil is being burned in heaters. While fuel oil may be the main source of fuel, considerable gas is still burned especially gas produced as a by-product in various processes in the refinery. Thus, the optimization control must be capable of handling various amounts of different fuels while still optimizing the combustion process.